The present invention is related to methods and apparatus for laying pipeline along the ocean floor and more particularly to such methods and apparatus for preventing buckling in large diameter thin walled pipeline that is assembled along the ocean surface and sequentially lowered to the ocean floor.
Utilization of underwater pipeline is becoming more prevelant with the advent of offshore oil and natural gas drilling. Oil or natural gas received from such drilling operations is usually either pumped into a tanker ship or piped under water along the ocean floor to a mainland refining facility. The pipeline utilized in the latter operation is often rather large in diameter, one example being a pipe of two feet inside diameter with a wall thickness of 5/8 inch.
It has proven to be an extremely difficult and expensive procedure to assemble and locate such a pipeline between an offshore facility and a distant facility whether it be an on shore refining facility or another offshore facility. The common procedure utilized is to obtain the pipeline in sections of equal length and connect them end to end to form the desired length. This procedure is accomplished on a specially designed barge that includes facilities for welding the pipe ends together and feeding the assembled lengths out as the barge moves from one location to another. Means is provided on such barges for applying a high tension to the pipe being fed out and lowered onto the ocean floor. Tension is maintained primarily to prevent buckling of the portion of pipeline extending from the barge to the ocean floor. The tensioning apparatus utilized ordinarily places from between 35,000 to 100,000 lbs. tension on the pipeline depending on water depth and pipe weight.
At the beginning of the operation, the beginning pipeline end is held stationary above the surface of the water or slightly below and the barge is moved away from that end as sections are added to the remaining end. Once a sufficient amount of pipeline has been assembled, the first (beginning) end is lowered to the ocean floor. A tug boat or other means maintains a constant tension on the section of pipeline extending between there and the barge until enough pipe has been lowered to the ocean floor to resist the tension without moving. Preferably, the length of pipeline laid on the ocean floor is sufficient alone to prevent the barge from pulling the first pipeline end along the ocean floor as it applies the required tension to prevent buckling. It may therefore be understood that a very significant length of pipeline must be fed out before it may be lowered to the ocean floor or additional means must be provided to prevent the first pipe end from being pulled along with the barge. The distance required if the pipeline length is held adjacent the ocean surface is often such that the pipeline cannot be maintained in sufficient tension to prevent it from swaying at the center and buckling as the pipe is initially being formed.
It is not infrequent that after the pipeline laying operation is well underway, the unsupported pipeline portion extending between the ocean surface and ocean floor buckles due to swells that cause the barge to change elevation relative to the ocean floor. The tension apparatus mounted to the barge is not capable of maintaining a continuous tension on the pipeline under such conditions and must therefore be set to apply a maximum amount of tension at the highest elevation the barge reaches in order to maintain a minimum amount of tension at the lowest barge elevation. Too often, the maximum amount of tension capable of being produced on the barge is insufficient to maintain the unsupported portion of pipeline under constant tension. The result is that frequently the pipeline buckles along the unsupported portion extending between the ocean floor and barge. Once the pipe buckles, a very tedious and extremely expensive procedure must then be followed to again take up that section of pipeline previously assembled and lowered in order to remove the buckled section and replace it with a new undamaged section.
An additional disadvantage of the tensioning process described above is that, in order to increase tension, an equal or greater pulling force must be provided by the barge to enable forward movement of the barge while maintaining such pipeline tension. This force is applied through means of a plurality of extremely heavy anchors. Ordinarily, approximately eight anchors are utilized to secure the barge against undesired movement and to enable the barge to pull itself along the ocean floor rather than being propelled by other means. The method utilized for propelling the barge is accomplished by one or two tug boats that lift and move anchors forwardly to distant locations while other anchors are being pulled inward by winch means on the barge. Obviously, if additional tension is to be placed on the pipeline, the anchor weight must be increased or the anchors be more efficiently and securely engaged with the ocean floor. Obviously, this again constitutes a tedious and expensive portion of the operating costs.
It is a primary objective of the present invention to provide a method and apparatus that will constantly tension the unsupported portion of pipeline extending between the ocean floor and ocean surface, independently of sea conditions and barge movement whether horizontal or vertical.
A further objective is to provide such a method and apparatus that will allow progressive flooding of that section of pipeline on the ocean floor.
A further objective is to provide a method and apparatus that will constantly tension the unsupported portion of the pipeline independently of tension on the forward barge anchors.
A yet further object is to provide such a method and apparatus that is significantly less expensive to operate, maintain and purchase, than any previously known method or apparatus.
An additional object is to provide such a method and apparatus wherein the capacity for tension is inherently increased geometrically as the pipeline radius increases.
An additional objective is to provide a method and apparatus to internally pressurize the submerged pipe in order to counteract the external hydraulic pressure created by increasing water depth.
These and yet further objects and distinct advantages will become apparent upon reading the following description which, taken with the accompanying drawings, describe a preferred form of my invention. It should be noted that the description and drawings are exemplary of a single embodiment of my invention and are not intended to restrict the scope of my invention as defined in the appended claims.